As a copper fine powder having a narrow particle size distribution, an average grain size ranging from 0.l .mu.m to a few .mu.m , and a spherical form has an excellent pasting property and finely forms a thick film conductor when used in an electronic circuit, and also a copper fine powder having a spherical form can be formed into a fired film having a high density, so that the electric resistance can be reduced, such copper fine powders having the properties as described above have been required.
Various processes have been known to prepare such copper fine powders, among which a liquid phase reduction precipitation process is employed as a method industrially practiced as well. This method comprises adding a reducing agent in a liquid phase containing copper ion and stirring the mixture to precipitate the metal powder directly in the liquid phase, and its examples include methods using reducing agents such as formalin (Japanese patent publication No. 76003/1980), hydrazine (Japanese patent publication No. 155302/1982) and sodium borohydride or dimethylamine borane (Japanese patent publication No. 224103/1983), and a method of reduction by hydrogen gas under pressurized condition (Japanese patent publication No. 22395/1968, Japanese patent publication No. 26727/1969), each of which affords a spherical or granular powder of several hundreds of .mu.m to a few .mu.m .
The copper fine powders prepared by those methods have considerably narrow particle size distributions and thus powders suitable for pasting can be frequently obtained, but the problems posessed by these methods are that a method having the better grain size and the better form controlling property requires the more expensive reducing agent and the production cost is high because of using a batch system reactor.
Also a method of reducing an oxide in a solid state has been known, but as this method generally results in large grain size and is affected by the form of the oxide, powders having the above properties are difficult to prepare.
Recently, a process for preparing superfine powders by means of a method of evaporation in gas or a fused metal reaction method using hydrogen arc plasma have been proposed, but these methods relates to superfine powders of a maximum of about 0.l .mu.m and have a disadvantage in which the powder is difficult to make into a paste when it is too superfine.
A process for preparing fine powders by means of a method of reducing a metal halide (vapor phase chemical reaction process, a kind of CVD) (Japanese patent publication 7765/1984) has been also proposed, but according to this method, the obtained powder is granular (in most cases, cubically shaped) in the fine powder zone of 0.l .mu.m or more. However, the vapor phase chemical reaction process has an advantage in which the used reactor is of a continuous system.